The present invention relates in general to radio receivers having a filtered RF input, and more specifically to AM/FM automotive radio receivers having an FM resonant input filter which conducts AM frequency signals substantially unattenuated.
Radio receivers must select a single broadcast from many strong and weak signals that may be present at the receiving antenna. The selectivity of a receiver refers to its ability to discriminate between a signal of interest and other signals at adjacent frequencies. This selectivity is typically achieved by bandpass filtering.
In a superheterodyne receiver, the radio-frequency (RF) signals are frequency shifted by mixing with an oscillator signal so that the RF signal of interest is shifted to an intermediate frequency (IF) which is typically lower than the RF frequency. A fixed IF filter is then used to reject signals at frequencies other than the IF frequencies so that filtering at the RF frequency stage is not necessary.
In order to improve sensitivity of a receiver, it is known to filter incoming signals at both the RF and the IF stages. The magnitude of signals of interest relative to other signals is increased. Furthermore, intermodulation distortion, which results from the mixing between signals not of interest into the frequency range containing the signal of interest, is reduced by providing RF filtering.
Resonant circuits typically provide the filtering used at RF frequencies. For example, a series resonant circuit located between an antenna and the input of a receiver can be provided having a controllable resonant frequency. A variable inductor connected in series with a capacitor are adapted to resonate throughout the FM frequency band. A tuning voltage generated in the receiver controls the resonant frequency of the series resonant filter to coincide with the signal of interest while attenuating other signals. This resonant circuit can be employed in combination with other series or parallel resonant circuits connected between the antenna and the receiver input or the receiver input and signal ground.
It is impractical to construct a series resonant filter for which the resonant frequency can be controlled to span both the FM and AM bands. Thus, when a series filter is used for an FM receiver in the prior art, signals in the AM frequency range are blocked by the series capacitor of the resonant circuit. Thus, a separate signal path would be required from the antenna to the receiver input for providing AM signals to the radio. However, most AM/FM automotive radio receivers comprise a single AM/FM antenna connected to a single receiver input using a single cable (i.e., there is only a single signal path) to reduce manufacturing complexity and cost.